Ash samples from tephra layers correlated with the Pomici di Avellino (Avellino Pumice) eruption of Somma-
Vesuvius were collected in distal archives and their composition and particle morphology investigated in
order to infer their behaviour of transportation and deposition. Differences in composition and particle
morphologies were recognised for ash particles belonging to the magmatic Plinian and final phreatomagmatic
phases of the eruption. The ash particles were dispersed in opposite directions during the two different
phases of the eruption, and these directions are also different from that of coarse-grained fallout deposits. In
particular, ash generated during magmatic phase and injected in the atmosphere to form a sustained column
shows a prevailing SE dispersion, while ash particles generated during the final phreatomagmatic phase and
carried by pyroclastic density currents show a general NW dispersion. These opposite dispersions indicate an
ash dispersal influenced by both high and low atmosphere dynamics. In particular, the magmatic ash
dispersal was first driven by stratospheric wind towards NE and then the falling particles encountered a
variable wind field during their settling, which produced the observed preferential SE dispersal. The wind
field encountered by the rising ash clouds that accompanied the pyroclastic density currents of the final
phreatomagmatic phase was different with respect to that encountered by the magmatic ash, and produced a
NW dispersal. These data demonstrate how ash transportation and deposition are greatly influenced by both
high and low atmosphere dynamics. In particular, fine-grained particles transported in ash clouds of smallscale
pyroclastic density currents may be dispersed over distances and cover areas comparable with those
injected into the stratosphere by Plinian, sustained columns. This is a point not completely addressed by
present day mitigation plans in case of renewal of activity at Somma-Vesuvius, and can yield important
information also for other volcanoes potentially characterised by explosive activity.